The federal antitrust suit, filed in Washington, D.C., argues that combining AT&T, the second-largest U.S. telecommunications company, with T-Mobile, No. 4, would reduce competition, drive up prices and mean less innovation for wireless consumers.

AT&T said the suit caught it off guard. T-Mobile representatives couldn't be reached for comment.

"We are surprised and disappointed by today's action, particularly since we have met repeatedly with the Department of Justice and there was no indication from the DOJ that this action was being contemplated," said Wayne Watts, AT&T's senior executive vice president and general counsel, in a statement. "We plan to ask for an expedited hearing so the enormous benefits of this merger can be fully reviewed. The DOJ has the burden of proving alleged anti-competitive affects and we intend to vigorously contest this matter in court."

Watts said AT&T believes that the "facts will guide any final decision" and that the facts weigh in its favor. AT&T's proposed purchase of T-Mobile, he said, would "help solve our nation's spectrum exhaust situation and improve wireless service for millions," and "allow AT&T to expand 4G LTE mobile broadband to another 55 million Americans, or 97% of the population."

The acquisition would enable AT&T and T-Mobile to create "tens of thousands of jobs, at a time when our nation needs them most," Watts said. Earlier Wednesday, Randall Stephenson, AT&T's chairman and chief executive, said the transaction would create about 5,000 jobs.